The ultrastructural analysis of morphologically discriminable fiber populations in extraocular muscle (EOM) of rabbit, mouse and rat will be continued, with emphasis on their respective topographical organizations and the differentiating aspects of their innervation. Dense serial sampling through individual neuromuscular junctions of the single end plate and multiple "en-grappe" varieties will be conducted to evaluate the extent and significance of preliminary findings that (a) the multiple endings may at times be much more complex in structure than previously thought, and (b) that end plates on different types of "fast" fibers exhibit different degrees of junctional complexity, in contradistinction to other descriptions in the literature. The initial finding that membrane-glycogen complexes in multiply innervated fibers are selectively distributed in restricted portions of these cells will be explored in detail. The structural changes in EOM evidenced by various animal models of inbred and drug induced dystrophies and myotonias will be investigated in time course studies, with emphasis on discerning primacies of disruption in the neuromuscular apparatus as apposed to the muscle fibers themselves. In both EOM and peripheral muscle comparison controls, the possible spreading of fiber disruption from the end plate will be accorded particular regard. BIBLIOGRAPHIC REFERENCES: Davidowitz, J., Philips, G., Pachter, B.R., and Breinin, G.M.: Particle-free and glycogen-bearing double membrane arrays in extraocular muscle of rabbit. Amer. J. of Pathol., 78: 191-198, 1975. Pachter, B.R., Davidowitz, J., and Breinin, G.M.: Muscle fiber and motor end plate involvement in the extraocular muscles of the myotonic mouse. Invest. Ophthalmol., 14:418-427, 1975.